1. Field of the Invention
The present invention relates to a method for cyclically measuring distances and velocities of objects using an FMCW radar sensor in which the frequency of a transmitted signal of the radar sensor is periodically modulated, each period including at least two differing modulation patterns, a relationship between distance and velocity of the object being derived from a signal received for a single modulation pattern, and the signals received for multiple modulation patterns being adjusted to one another in order to determine one value each for distance and velocity per each measuring cycle.
2. Description of the Related Art
Radar sensors are used in driver assistance systems for motor vehicles, e.g., in ACC (adaptive cruise control) systems or safety systems, e.g. collision warning systems, and are used to determine the positions and the relative velocities of objects, so that a driving situation may be recognized. For example, the velocity of the host vehicle may be adapted to the velocity of a preceding vehicle in an ACC system as part of an adaptive cruise control, and the distance from the preceding vehicle may be controlled to a suitable value. A safety system may, for example, be configured to automatically initiate a braking operation when a risk of an imminent collision has been recognized.
FMCW (frequency modulated continuous wave) radar sensors are known in which the transmission frequency of a continuous radar signal is modulated in a ramped manner. By mixing a received signal with the transmitted signal, a baseband signal is generated which is subsequently evaluated.
In an FMCW radar sensor, each radar object is plotted in the frequency spectrum of the baseband signal in the form of a peak whose position is a function of the Doppler shift and the propagation time of the radar signals, so that it is not yet possible to unambiguously determine the relative velocity and the distance from the baseband signal assigned to a frequency modulation ramp. The frequency of an obtained peak rather establishes a relationship between the velocity (relative velocity) and the distance in the form of a linear correlation.
The term “linear” is to be understood to mean in the following that the correlation designated this way may include a linear factor and an additive term.
In the case of the FMCW method, multiple frequency modulation ramps having different ramp slopes are necessary for identifying multiple radar objects and for estimating their relative velocities and distances. By adjusting the different received relationships obtained for the individual frequency ramps, relative velocity v and distance d of a radar object may be computed. This adjustment is also referred to as matching and corresponds to a search for points where the straight lines in the d-v space intersect. The FMCW method is particularly efficient if only a few radar objects are detected.
Radar sensors are also known which work according to the method of the chirp sequence modulation in which the transmitted signal includes a sequence of identical frequency-modulated signal pulses (chirps). The modulation pattern therefore includes a complete set of consecutive chirps and not of a single modulation ramp. This is a pulse Doppler method in which radar objects are initially separated according to their distances, and subsequently location changes and thus velocities of the radar objects are ascertained based on the differences in the phase positions between the reflections of the individual signal pulses.
In known FMCW methods, the cycle time, i.e., the duration of a single measuring cycle, matches the cycle duration of the frequency modulation. A certain number of frequency patterns is transmitted within one measuring cycle and the received signals are recorded and evaluated for all received frequency patterns. The cycle time is therefore composed of the time which is needed to transmit the frequency pattern and an additional computing time which a processor requires for processing the received signals and for computing the distance and velocity data.
In the case of safety-relevant assistance functions, it is important, however, that the traffic events may be tracked with a preferably high time resolution. This means that the cycle time should be as short as possible. Since it is not possible to shorten the duration of the modulation patterns for measurement accuracy reasons, the cycle time may only be shortened by shortening the computing time. The utilization of more powerful and thus more expensive processors is necessary to accomplish this.